As the demand for data bandwidth increases, so does the demand for high bandwidth optical data transmission techniques.
Typically, there are two basic ways that digital data is formatted in fiber optic systems. The two formats are the return-to-zero (RZ) format and the non-return-to-zero (NRZ) format. In the NRZ format, each bit of data occupies a separate timeslot and is either a binary 1 or a binary 0 during that time period. In contrast, in the RZ format, a time period is allowed for each bit. Each bit is transmitted as a pulse or an absence of a pulse. Both formats are referenced to a system clock. The system clock, however, is not a separate signal and must be recovered from the data. A clock signal may be recorded, for instance from NRZ data, by using the transition occurrences within the data transmitted. The process of recovering a clock signal from transmittal data is typically referred to as clock data recovery (CDR). Clock data recovery subsystems are a key block for digital communications and telecommunication circuits. CDR systems are also used in a variety of other digital systems, for example disk drives.
Commonly clock data recovery circuits are based on the use of phase lock loops (PLL). Unlike phase lock loops that are used in wireless applications, a CDR PLL operates on random data, such as but not limited to non-return-to-zero data, instead of a sine wave or modulated sine wave signal. With NRZ data, the clock signal, which is encoded with the data, must be regenerated from the data since the data must eventually be processed synchronously. A further complication with clock and data recovery circuits is that the data spectrum is broadband. This is in contrast to the narrow band spectrum PLLs, which are commonly encountered in typical PLL applications such as synthesizers, demodulators, and modulators.
In CDR circuits, a regenerated clock signal is typically used to retime the data through a Flip-Flop, which is used as a decision circuit. This retiming of data comprises the data recovery function of the CDR circuit. By retiming the data, the data stream is essentially recreated and time domain jitter, which may be present in the NRZ signal or produced by the NRZ receiver circuitry, may be greatly reduced.
A typical application using clock and data recovery circuits is a SONET (synchronous optical network) system. In SONET systems, the CDR subsystem has difficult performance specifications to meet in terms of jitter tolerance, jitter generation, jitter transfer, bit error rate, and phase margin. These performance specifications are held within tight tolerances so that SONET systems may deliver high quality data with a low BER (bit error rate).
A key parameter affecting the quality of data received is the phase margin. Phase margin is the phase relationship between data and clock that results in correct data being reproduced. In other words, if the phase margin of a decision circuit that is decoding the transmitted data needed is exceeded, the probability that errors can result may increase. In order to minimum phase margin error, the clock should cause the data to be sampled at times when the data is stable, that is, at a time when the data is not in transition. Such sampling requires that the sampling edge of the clock signal reside at or near the middle of the transmitted data bit. This condition, in which the clock resides in the middle of the data bit, is referred to as centered clock/data. To achieve the condition of centered clock/data the phase lock loop within the clock and data recovery circuit must maintain a particular static phase offset between the clock and data. This static phase offset requirement is typically more stringent than the lock requirement in standard PLL applications. In addition, because the clock regeneration is using a data stream to regenerate the clock, the performance of the phase detector will be dependent on the data patterns within the data transmitted.
Commonly Hogge type phase detectors are used in clock data recovery circuits. Process, temperature, voltage, data pattern, transition density, and matching circuit delay variations affect the performance of Hogge type phase detectors. Such variations, which are difficult to compensate, result in a combination of increased static phase error, reduced phase margin, and high pattern dependant jitter. In high-speed designs, the effect of such variations is exacerbated. Accordingly, design issues become more critical for proper circuit operation as data rate increases.